“Son of Sam” serial killer David Berkowitz was so terrified of snitching on the man who slashed his throat in the slammer 36 years ago that he claimed he merely ran into a window, a former prison official told The Post on Monday.

Berkowitz blamed himself as blood gushed through his fingers from a gaping wound that was clearly not self-inflicted, said James Conway, a retired superintendent at the infamous Attica Correctional Facility upstate.

“Berkowitz said, ‘I wasn’t paying attention and I ran into the window,’ ” recalled Conway, who was an Attica correction officer at the time of the July 10, 1979, attack.

That explanation was immediately dismissed because the gruesome injury snaked from Berkowitz’s chin to the back of his head.

On Monday, The Post revealed that career criminal William Hauser was the suspect.